I'm not sure what you mean by implement these in Perl. Do you want to do something like this:

assoc('.pl', 'd:\perl\bin\perl.exe');
# so you can run programs like this:
run_cmd('c:\stuff\script.pl');
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If this is what you're looking for, you could make a hash of known file extensions. Your functions could look like this (untested):

use vars qw/%extensions/;

# keep known extensions here.
%extensions = ();

sub assoc {
    my ($extension, $prog) = @_;
    $extensions{$extension} = $prog;
}

sub run_cmd {
    my $cmd = shift;
    my $ext = "";

    if ($cmd =~ /(\.\w+)$/) {
       $ext = $1;
    }

    # prepend interpreter for known extension
    if ($ext and exists $extensions{$ext}) {
        $cmd = "$extensions{$ext} $cmd";
    }

    return `$cmd`;
}
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This loose code won't support arguments like myscript.pl --args, but that can be left as an exercise in string parsing.

I hope this is what you were asking!

Update: John M. Dlugosz's response above is probably more what you're looking for, upon reinspection of your question.